Myeloma is unique among the hematologic malignancies in its capacity to cause bone destruction. However, bone-resorbing cytokines that are responsible for the bone destruction in patients have not been identified. Recently, we found that myeloma cells in culture produce low amounts of bone-resorbing cytokines unless cocultured with marrow stromal cells or treated with soluble VCAM-1 receptor which mediates cell-to-cell interactions between myeloma cells and stromal cells. These results indicate the importance of the bone environment in the behavior of myeloma. Based on these observations, we hypothesize that: (1) Myeloma cells produce excess bone-resorbing cytokines when in direct contact with marrow stromal cells; (2) This cell- to-cell interaction is mediated by VCAM-1 expressed by marrow stromal cells and by alpha4Beta1 integrin expressed by myeloma cells; and (3) The production of these cytokines is important not just for the bone destruction, but also for further progression of the malignancy. To test these hypotheses, we will: (1) Determine the importance of the cell attachment molecules VCAM-1 and alpha4Beta1 integrin in vivo in our murine and human models of myeloma bone disease using neutralizing antibodies we have available to VCAM-1 and to alpha4beta1. In addition, we will determine in bone sections from our models of myeloma bone disease and in marrow biopsy sections from patients with myeloma, if myeloma cells and marrow stromal cells bind via VCAM-1 and alpha4beta1 integrin using immunohistochemical analyses with antibodies against these moieties that react with both human and murine VCAM-1 and alpha4beta1; (2) Determine the role that Rank ligand plays in myeloma bone disease; (3) Determine the known cytokines expressed by murine myeloma cells cocultured with stromal cells or by VCAM-1 treated myeloma cells by treating the conditioned media with neutralizing antibodies to osteoclast- activating cytokines. If osteoclastogenic cytokines are identified which we cannot neutralize with antibodies to the known cytokines, we will clone, identify and characterize these unknown bone-resorbing cytokines using an expression cloning approach.